This invention relates to oscillators and particularly to multi-mode lasers in which the mode of oscillation is selected by an optical input signal for the purposes of contrast enhancement of projected optical images and performing threshold operations on optical matched filter output images.
Contrast enhancement is a basic image processing operation in which low image values (e.g., low intensity) are transformed to lower values and high image values are increased so as to render objects more easily visible when depicted in low contrast. In an extreme case, all picture values less than a certain threshold value are reduced to zero and picture values greater than this threshold are increased to the maximum possible value. This is called a "threshold operation" and finds extensive use in sharpening blurred images of printed characters and in optical information processing. Contrast enhancement has been carried out in prior art image processing by many devices and techniques including photographic methods based on special high contrast films and developers, and video methods based on manipulations of the electrical signals produced at the output of optical detectors.
In order to describe the use of threshold operations in optical data processing, it is desirable to first briefly review the uses of optics in data processing. For this purpose, only those computing methods in which optics performs the bulk of the computations need be discussed. The uses of optics for specialized tasks such as input-output, memory, etc. are omitted.
Optics has been used in data processing in two quite distinct ways. In the first branch of optical computing, computations are effected when optical beams which represent numerical quantities analogically are passively combined, attenuated or altered in phase. Typically, these operations are organized in a highly parallel fashion. Each point of an optical image may be processed simultaneously to achieve an extremely high data rate. In some schemes, the attenuation of the information-bearing beams may be controlled by a photochromic absorber which alters its optical density in response to a second light beam. An example of this art is given by C. Carlson in U.S. Pat. No. 3,085,469.
Although incoherent light can be used in these representations, the most effective devices require the use of coherent light. A recent summary of results in this field has been given by L. Cutrona, IEEE Spectrum, Vol. 1, p. 101, Oct. 1964. Particularly significant is the use of holographic coherent optical techniques to synthesize complex valued spatial filters. Holograms have been described by R. Collier in an article in IEEE Spectrum, Vol. 3, p. 67, July 1966. In spatial filtering, an image representing the cross correlation of an input image with a fixed filter response function is formed. The bright points of this image with intensity exceeding a certain threshold value indicate the existence and location of specific patterns in the input image. These bright points can be determined by application of a threshold operation to the correlation image. If the spatial filter is a matched filter, the brightest point in the output image indicates the most probable location of the matched signal. The brightest point can be located by a series of threshold operations of decreasing level, decreasing to that threshold value which is exceeded by just one point.
Since these coherent light devices are passive and typically linear, they are very limited in the types of data processing operations which they can perform without conversion to another media. Use of photographic film precludes real time operations and conversion to an electrical format causes loss of parallelism severely reducing data processing rates. Technically, this first branch of optical computing is greatly concerned with diffractive processes since the manipulation of coherent light to obtain the extremely high resolution and data storage density is strongly affected by diffraction. High resolution promotes high data processing rates since each resolvable point of the input optical image is processed in parallel as a separate channel.
The second branch of the optical computing art uses optics to control the generation of light so as to effect computational processes. Since this control typically results in an amplification of optical energy, the devices used in this branch are said to be active. Incoherent light can be used; however, the most powerful techniques involve the control of the generation of light emitted by lasers. These coherent light sources were first described by A. Schawlow and C. Townes, Physical Review, Vol. 112, No. 6, p. 1940, Dec. 1958. Lasers consist of an active optical element placed in an optically resonant cavity. When suitably pumped, the active element becomes capable of amplifying optical radiation thus compensating for losses in the cavity. Optical oscillations ensue which produce the laser output as they are emitted from the cavity. Typically, a laser output is of a particular sharply defined optical frequency, i.e., temporally coherent. A general summary of laser technology may be found in the book Laser-Light Amplifiers and Oscillators by D. Ross, published by Akademische Verlagsgellschaft, Frankfurt am Main, 1966. A later English edition is also available.
Prior art devices in this branch do not have a high degree of parallel action. Usually, each separate device processes only one information channel and is expected to achieve a significant computational result only when many thousands of such devices are combined or when each acts sequentially in a serial fashion. Furthermore, since lasers tend to be bistable devices (either all on or all off), most applications in this branch are adapted to a digital form of computation in which numerical quantities are represented by several optical channels each realizing one binary bit as "0" or "1" according to whether it is "off" or "on".
Some recent results in this branch of optical computing have been given by C. Koester and also by W. Kosonocky both in the book Optical and Electro-Optical Information Processing, ed. by J. Tippett, et al, published by M.I.T. Press, 1965. Koester shows how the generation of laser light may be controlled optically by controlling the gain of the active element. Kosonocky shows how the control may be achieved by affecting the losses in the cavity. He places a saturable absorber in the cavity of sufficient optical density to inhibit laser oscillation. A saturable absorber is an absorbing material which can be bleached by light action. An optical input signal over a certain threshold intensity bleaches the saturable absorber, reduces attenuation and allows the laser to oscillate. In some cases, the lasers were allowed to have 2 or 3 distinguishable types of laser oscillation (modes) in the same body each mode acting as a separate independent light source. The reference shows how the oscillations of these modes can be controlled separately by a combination of the techniques mentioned above. Since the controlled light is usually much brighter than the controlling light, an input-output gain greater than unity is usually achieved.
Since the control of light by alteration of the attenuation of a saturable absorber plays an important role in this disclosure, a more detailed explanation of the usefulness of the saturable absorber in the Kosonocky device will be given here. It has already been mentioned that laser mode oscillations are basically bistable and hence oscillation can be theoretically controlled by injecting light into a non-oscillating mode and increasing the intensity until oscillation begins. However, in order to get this behavior, the injected radiation must match the stimulated mode exactly in frequency, polarization and spatial distribution. The match in frequency is particularly difficult because it must be within the width of one resonant cavity mode. Since the frequencies of cavity modes are substantially changed by the slightest variation in cavity parameters, it is not even sufficient to use light from another laser of the same type. For these reasons, mode control by mere injection of radiation has not been reduced to practice. The introduction of the saturable absorber solves the problem since the bleaching radiation need not be related in any way to the laser mode frequency. The saturable absorber also serves to force the state of oscillation to be bistable since it is bleached completely as soon as the laser begins to oscillate and the intensity of oscillation rises to its maximum value.
It has been seen that threshold operations are involved in both branches of optical computing. In the first branch, a threshold operation is needed to indicate high correlation values and indicate pattern recognition. Yet the equipment itself, with its passive parallel nature, cannot supply this action. In the second branch, the threshold action is inherent in the laser devices themselves, yet it is used only to enforce a bistable action on laser oscillation to mark a "0" or a "1" in digital serial systems with little parallel computing capability.
The divergence of the two branches of optical computing is clearly evident on comparison of the article by L. Cutrona with the article by T. Bray, both of which appear in the book Optical and Electro-Optical Information Processing previously referred to. The article by Bray deals with the gain and the smallness of the optical elements but nowhere mentions resolution or diffraction. The article by Cutrona is concerned mainly with diffraction by large passive elements and the laser appears only as a steady source of coherent light.
Numerous advantages of basing entire computational systems on optics have been cited, for example by O. Riemann in the book referred to in the previous paragraph. Bray also gives several advantages of an all-optical approach, especially the parallel action mentioned above. However, later studies have concluded that optics could be of use in general purpose computers only in specialized roles such as input-output, memory, etc. See, for example, the article by W. V. Smith in Applied Optics, Vol. 5, No. 5, p. 1533, Oct. 1966. A possible reason for failing to achieve the advantages of all optical information processing is the lack of optical computing devices which combine the advantages of parallel processing with the controlled amplification of light (i.e., which merge the two branches of the optical computing art) and which manifest the threshold action needed by the first branch and suppliable by the second.
Some devices have been described which tend to bridge this gap. They involve control of laser generated light in a large number of separate channels. Each channel is realized by a separate mode of a multi-mode laser. As explained by Schawlow and Townes, op. cit., optical cavities naturally oscillate in many thousands of distinct modes distinguishable in frequency or direction of propagation or both. Commercially available lasers which serve as coherent light sources oscillate in only one or a few modes because all others (but these few modes) have been suppressed by artfully reducing their gain. In optical scanning, it is desirable to have multi-mode lasers which oscillate equally well in a large number of independent modes, each mode acting as a separate source for one direction of a scanning beam. For this purpose, the modes should be degenerate, i.e., have equal gain.
Multi-mode lasers can also be used as image amplifiers when each point of the image is coupled into a separate mode of a suitably pumped laser. In discussing image amplifying lasers, modes are considered distinguishable only if they have spatially distinguishable wave fronts of different shape or different direction. Modes of the same shape and direction are grouped together since they amplify the same image point. Even with this relaxation of the mode definition, such image amplifiers tend to be impractical since they require precise injection into a cavity mode if substantial gain using repeated (resonant) passage through the active material is to be achieved.
A practical image amplifying laser is described by W. Hardy in the IBM Journal of Research and Development, Vol. 9, No. 1, p. 31, Jan. 1965. In this laser, each mode of a multi-mode laser separately amplifies one point of an optical image as given by a transparency placed in the cavity. The intensity of the oscillation of the mode associated with a given point increases with the transmission of that point. This is called active optical imaging. The output of this device is given in terms of optical radiation but the input is a transparency.
A similar but more powerful active imaging device was described by R. Meyers, et al, in IEEE Journal of Quantum Electronics, QE-10, p. 270, Aug. 1966. The same laser has been used by R. Pole in a laser scanner in which the direction of the scanning beam is determined by the selection of the mode of oscillation which is controlled by an electro-optic modulator in the cavity. This scanner is described in the book Optical and Electro-Optical Information Processing previously referred to. In Applied Optics, Vol. 6, p. 1571, Sept. 1967, Pole has shown how an optical data processing system called "reactive information processing" can be implemented by placing a transparency in the multi-mode cavity mentioned above. In this system, the modes of the laser which are influenced by the information input are non-oscillating modes so this approach leads back to the passive coherent optical computing branch previously discussed.
These laser devices of Hardy, Meyers and Pole (intermediate between the two branches of optical computing) are active in nature, have parallel computing organization and have optical radiation as the output but, since they do not have optical radiation as an input, they cannot provide a threshold action on optical beams or projected optical images obtained from passive coherent optical data processors. Nor can they be combined with each other to form all optical computing systems because their input and output signals are incompatible.